Ospho and similar rust treatments contain phosphoric acid, not good for prolonged contact with skin. If POR-15 is not exposed to UV, no need to top coat. Not much sticks to galvanized metal for an extended period of time. If you get the POR-15 it will be there until it wears off. Don't ask me how I know. The rusted parts of the galvanized metal can be treated.

Bluto, I am the third owner. The Airstream originated in New York for one year then purchased by previous owner and was purchase by me from a widower after setting for three years. I suspect damage was caused by road salt spray and never addressed. There is some surface corrosion to the frame and axles, but not as extensive as pictures. It is a possibility that rusted parts were top coated from factory having worked in the auto industry for 30 yr. Quantity over Quality. Also have some filiform corrosion, mostly lower part of battery compartment frame. I did find a company to order stainless replacement latches at a reasonable price for the severely rusted ones on the compartment doors. I do need to figure out pop rivet sizes. Happy to be a part of your community.

Wow - I got more time now, so I'll lend some opinions but also ask for some more pictures.

If you want the powder-coat smooth finish for exterior stuff I can't help you - the POR-15 is paint-over-rust-15, it wants the surface rough so it will key in. First coat fills the pores and the second coat applied while still gummy provides the armored finish. The texture will be still there though they say the aluminum filled color has the most solids to build up heavy rust pits. I used the semi-gloss and am very happy with it, even exposed for two years now it just has a powdery surface dust on it and nothing rusting on the hitch tongue or bumper brackets, etc. Ospho or other chemical treatments are not necessary. I painted over outrigger iron that had corroded to lace and its just like the day I painted it, exposed to sun and weather every day since.

Crawling under the frame and using wire wheels and scotch-brite biscuits on a die grinder is a lot of fun. The original paint gets cleaned and top coated with rustoleum or similar where its not damaged and the active rust areas get POR-15. Health hazards for dust and fumes are present but nothing out of the usual for OTC sales to the public.

There are encyclopedic postings here on filigree corrosion - my old trailers don't suffer from it.

Large flange rivets for the belly pans. Olympic rivets if you have to drill out the bucked factory rivets, and various length 1/8th shank rivets for trim and such - one important point here is to get 1/8" rivets with the aluminum mandrel 'nail' part so there will be no rust spot or stains as it weathers.

On the galvanized - the sanitary holding tank contents are acidic, think it had contact with any leakage? Was it rusting from inside out or outside in? Has the metal perforated? If its from the inside out there was just a posting "inexplicable bumper design" that discussed coatings and things. Truck bedliner coating may be a valid top coat to prevent it from happening again (will it bond to POR-15?)

8wheeler,
Thanks and good luck with your project. Looks like a lot of work!
If it were me, and my budget allowed, I'd check out a trailer grave yard for boxes that are in better shape and just replace parts as needed.

Thanks Wabbiteer for the help and results of your experience. The rust in the galvanized trunk occurred from under a rubber mat placed there and water leakage from around the bumper never able to completely dry. The under side of the trunk does have a large spot of white rust. I will be glad to post additional pictures. I do plan on stripping the boxes so would prep work be required for the POR-15 on them and I plan on top coating everything with a Rustoleum type product. Suggestions for color?
There is some light surface rust on underside of galvanized holding tank covers. Should I drop those and check for leaks? This project is growing,but I am willing to put the time and effort in to do it right and share my experience. This is a great forum and one reason I purchased an Airstream although one with some concerns and not especially visually appealing on the underside for its young age.